Printed Coplanar Two-Antenna Element for 2.4/5 GHz WLAN Operation in a MIMO System

PRINTED COPLANAR TWO-ANTENNA
ELEMENT FOR 2.4/5 GHZ WLAN
OPERATION IN A MIMO SYSTEM
Saou-Wen Su,1 Jui-Hung Chou,1 and Yung-Tao Liu2
1
Technology Research Development Center, Lite-On Technology
Corporation, Taipei 11492, Taiwan
2
Department of Physics, Republic of China Military Academy,
Fengshan 83059, Taiwan; Corresponding author:
stephen.su@liteon.com (a)

Received 24 October 2007

ABSTRACT: A printed coplanar two-antenna element suited for WLAN
operation in the 2.4- and 5-GHz bands for dual-module applications in
a MIMO system is proposed. The two-antenna element is comprised of
one planar inverted-F antenna (PIFA) and one monopole antenna, both
printed and integrated in a coplanar configuration on a narrow dielec-
tric substrate with the dimensions 50 mm 11 mm. The two antennas
are excited using two separate feeds with a common ground plane. By
utilizing two proposed elements spaced 10-mm apart in the lateral di-
rection with an optimized arrangement, the four-antenna MIMO system
obtained can achieve optimal isolation between any two of the four an-
tennas. In comparison with the conventional dual-band antenna with a
single feed, the proposed two-antenna element allows the 2.4- and
5-GHz signals to be simultaneously received or transmitted with no ex-
ternal switch circuit between the antenna and modules required. © 2008 (b)
Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 1635–1638,
2008; Published online in Wiley InterScience (www.interscience.wiley. Figure 1 (a) Detailed dimensions of the printed coplanar two-WLAN-
com). DOI 10.1002/mop.23446 antenna element. (b) Photo of a mass-production sample for the proposed
two-antenna element. [Color figure can be viewed in the online issue,
which is available at www.interscience.wiley.com]
Key words: printed antennas; monopole antennas; PIFAs; WLAN/
MIMO antennas
2. PRINTED COPLANAR TWO-ANTENNA ELEMENT

1. INTRODUCTION Figure 1(a) shows the configuration of the proposed two-antenna
element. A photo of a working sample of the antennas for mass
Multiple-input multiple-output (MIMO) technology using multiple
production is demonstrated in Figure 1(b). The two antennas,
internal antennas has been applied to mobile devices to increase
comprised of one monopole antenna and one PIFA designated for
data throughput [1–3]. For applications of access points, very few
2.4- and 5-GHz operation, respectively, are all printed on the same
studies are conducted on possible internal MIMO antennas. In fact, layer of a 0.4-mm-thick FR4 dielectric substrate (size 50 mm 11
external dipole and/or monopole antennas are still employed gen- mm) and share a common antenna ground. The two antennas are
erally in the market for MIMO systems, which are not pleasing to located at opposite ends of the ground plane, whose length can be
the end user from an esthetic view point. In addition, most of the favorable to good impedance matching and also affects the isola-
designs are with a single feed for generating two resonant modes tion between the antennas in both the 2.4- and 5-GHz bands. The
for covering the 2.4- and 5-GHz bands. For current MIMO module antenna-ground length of 30 mm is chosen in this study. Note that
applications, two separate modules for the 2.4- and 5-GHz oper-
ation are used. Thus, external switch circuit is required between
the single-feed dual-band antenna and the modules, leading to an
increase in the insertion loss over the board. In this letter, we
present a printed two-antenna element with two feeds for MIMO
system suitable to be concealed in a wireless device for dual-band
WLAN (2400 –2484/5150 –5825 MHz) operation. The two-ele-
ment antenna has one PIFA and one monopole antenna. Both the
antennas and the ground plane are printed on the same layer of a
substrate in a coplanar configuration [4]. The isolation between the
two antennas can be decided by the ground-plane length. Further,
by combining two proposed two-antenna elements, a four-antenna
MIMO system is obtained. Different arrangements of the two
elements will be elaborated and analyzed, especially in the isola-
tion behavior, in the Section 3. Radiation properties for a single
two-antenna element and the combination of the two elements are
also studied. Details of the two-antenna element and the four- Figure 2 Combinations of the two proposed two-antenna elements in a
antenna MIMO system are described, and the results are presented MIMO system. [Color figure can be viewed in the online issue, which is
and discussed. available at www.interscience.wiley.com]

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 50, No. 6, June 2008 1635

two symmetrical slits are cut out of the monopole antenna to MIMO system in the next section are based on the fixed value 10
meander the excited surface currents, which leads to a compact mm for the distance d. The results of the radiation characteristics
size for the 2.4-GHz antenna. For testing the two-antenna element between the two-antenna element with one antenna excited and the
in experiments, two short 50- mini-coaxial cables with I-PEX two proposed elements with two antennas excited will be analyzed
connectors are utilized. The inner conductor of the coaxial cable is too.
connected to the feeding point (points A and C) and the outer
braided shielding is connected to the grounding point (points B 3. RESULTS AND DISCUSSION
and D).
Figure 2 shows three possible arrangements, denoted as com- 3.1. The S Parameter Analyses
binations 1 to 3 here, of the two proposed two-antenna elements. Figures 3(a) and 3(b) show the measured and simulated reflection
Notice that the configuration presented in Figure 1(a) is for com- coefficients (S11 for the 2.4-GHz antenna, S22 for the 5-GHz
bination 1 arrangement. The two proposed elements are spaced antenna) and isolation (S21). It is first noticed that in general, the
apart at a distance of d in the lateral direction. That is, both the experimental data compare favorably with simulation results,
shorter edges of the two elements are facing each other. In this which are based on the finite element method using Ansoft HFSS.
case, a low-profile MIMO system having four antennas is formed. The impedance matching in both the 2.4- and 5-GHz bands is
The obtained MIMO system has a height of 11 mm and can be much less than 10 dB, and the isolation between the antennas
installed in some space inside the housing of a wireless device. The remains under 15 dB over the operating bands. Also notice that
distance d plays an important role in influencing isolation behavior a higher resonant mode is found at about 3.4 GHz, which is mainly
in the two proposed elements. A larger d can result in better attributed to a dipole-like resonant mode well excited in the
isolation in general, which have been recently reported with sim- monopole antenna. In this case, the simulated surface currents in
ilar interests [5, 6]. Between any two of the four antennas, the the antenna and the ground plane are observed to be distributed in
isolation of less than 15 dB, which is usually demanded for the opposite direction with null currents existing at one third of the
practical applications in industry, can be achieved when the dis- ground plane from the monopole antenna. On the other hand the
tance d is larger than 10 mm. The studies for the four-antenna current distribution is found to be in the same direction in both the
antenna and the ground plane when the antenna operates in the
monopole resonant mode at about 2.42 GHz.
The isolation behavior between the antennas of the two ele-
ments in the MIMO system for combinations 1 to 3 was also
studied, and the results are shown in Figures 4(a)– 4(c). For com-
bination 1, it is first seen that the isolation is all less than 15 dB
within the bands of interests. Isolation between the same antennas,
i.e. the antennas 1 and 3, is the poorest in the antenna’s operating
band, i.e. the 2.4-GHz band, as the characteristic is reasonably
expected. Also worth noticing that the isolation S32 is not better
than S41 even though the distance between the antennas 2 and 3 is
much longer than the distance between the antennas 1 and 4. The
phenomenon is largely because the factor in the influence of
isolation in this four-antenna MIMO system is decided more by
antenna radiation patterns (pattern diversity) than by the separation
distance (spatial diversity) [7]. The maximum radiation of the
5-GHz PIFA has been found to be in the direction of the antenna
(a) ground (see inset in Fig. 5), thus leading to much degraded
isolation for the antennas 2 and 3.
For combinations 2 and 3 in Figures 4(a) and 4(b), the worst
isolation is seen with the two same antennas placed adjacent to
each other. The isolation S41 and S32 are about the same due to the
same arrangement for the antennas 1, 4 and 2, 3. The isolation S42
in the 5-GHz band is not better in combination 2 than in combi-
nation 1, even the combination 2 with a longer separation distance
between the two PIFAs. The results also indicate that the isolation
property in this MIMO system can be affected more by the
radiation patterns (the maximum radiation of the 5-GHz antennas
in combination 2 facing each other) than by the separation dis-
tance.

3.2. The Radiation Characteristics
Figures 5(a) and 5(b) plot the measured and simulated 3D radiation
patterns for the two-antenna element studied in Figure 1 at 2442
(b) and 5490 MHz, the center operating frequencies of the 2.4- and
5-GHz bands respectively. The far-field measurement was con-
Figure 3 Refection coefficients (S11 for the 2.4-GHz antenna and S22 for ducted at a 3 3 7 m3 anechoic chamber at Lite-On Technol-
the 5-GHz antenna) and isolation (S21) between the two antennas: (a) ogy, Taipei. First, the measured and simulated results are not
measured results; (b) simulated results. [Color figure can be viewed in the appreciably different. The radiation characteristics for the 2.4-GHz
online issue, which is available at www.interscience.wiley.com] antenna here resemble those of the dipole antenna, in which

1636 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 50, No. 6, June 2008 DOI 10.1002/mop

(a) (b)

(c)

Figure 4 Simulated S-parameters (S31, S41, S32, S42) for the two proposed elements in the four-antenna MIMO system in Figure 2 with d 10 mm: (a)
for combination 1; (b) for combination 2; (c) for combination 3. [Color figure can be viewed in the online issue, which is available at www.interscience.
wiley.com]

omnidirectional radiation is obtained in the horizontal cut (the x–y
plane in this case). That’s because the ground-plane length corre-
sponds to a quarter-wavelength at around about center operating

(a)

(a)

(b) (b)

Figure 5 The 3D radiation patterns at 2442 and 5490 MHz for the Figure 6 Measured peak antenna gain and measured radiation efficiency
antennas studied in Figure 3: (a) measured results; (b) simulated results. for the antennas studied in Figure 3; (a) for the 2.4-GHz antenna; (b) for the
[Color figure can be viewed in the online issue, which is available at 5-GHz antenna. [Color figure can be viewed in the online issue, which is
www.interscience.wiley.com] available at www.interscience.wiley.com]

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 50, No. 6, June 2008 1637

4. CONCLUSION
A printed two-antenna element with two integrated internal anten-
nas for 2.4- and 5-GHz WLAN operation has been introduced,
fabricated and tested. A four-antenna MIMO system formed by
utilizing the two proposed two-antenna elements has been studied.
Various possible arrangements of the two proposed elements for
achieving optimal isolation have been analyzed too. The results
indicate that between any two antennas of the two elements in the
MIMO system, maximum isolation is less than 15 dB for oper-
ating frequencies over both the 2.4- and 5-GHz bands. Good
(a) radiation characteristics of the antennas have been observed. It has
been found that the antenna radiation pattern is also an important
factor in the influence of isolation in the MIMO system here. The
proposed two-antenna element is well suitable for internal WLAN
antennas in a MIMO system, especially for applications in a
wall-mount access point.

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Total radiation of the two proposed two-antenna elements was
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also studied, and the results are shown in Figure 7. When the
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compared with that of the 2.4-GHz antenna in a two-antenna © 2008 Wiley Periodicals, Inc.
element studied in Figure 5(b). As for the two 5-GHz antennas in
Figure 7(b), more side lobes in the elevation planes can be seen in
the total radiation but still with the maximum radiation in the z
direction, which is the same direction for the 5-GHz antenna
shown in Figure 5(b). The combined antenna gain in the horizontal
plane is also larger here than that for a single 5-GHz antenna
excited in the two-antenna element. Increase in the peak antenna
gain by about 2.6 dBi has been observed, too, when both the
5-GHz antennas operate in the proposed two-antenna elements.
The results of gain enhancement suggest that the array effect of the
two elements is also had in the MIMO system in this study.

1638 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 50, No. 6, June 2008 DOI 10.1002/mop

Printed Coplanar Two-Antenna Element for 2.4/5 GHz WLAN Operation in a MIMO System

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